Right from the time kids are babies, they're getting dressed up for Halloween (although for babies the thrill is more for the parents than the kids!)
There are almost unlimited ideas for kids costumes, but the most popular from one year to another are the licensed characters from their favorite movies and TV shows. And usually the most popular is any character from the past summer's biggest hit movies.
More tried-and-true costumes like pirates, witches, ghosts and aliens always work too. As kids get older, they usually get gorier every year. Fake wounds, scars and blood are common sights with older trick-or-treaters. And the less their parents like it, the better.
Grown-up Halloween costumes are getting more popular every year as well. There are tons of licensed costumes for adults as well as many of the old standby's.
When adults get dressed up, they quite often like to stick to a theme - everyone at the party dresses according to theme or perhaps a couple will dress in matching costumes of some kind.
Historical costume like Renaissance or Medieval outfits are also popular with grown ups. Or "decade" costumes - flappers from the Roaring 20's or a 40's zoot suit.
One difference between kids costumes and adults is that adults often put together their own outfit, with more attention to the details. Kids are perfectly happy with a costume that you bought in the local drug store.
Adult costume can get quite detailed - something that would be right at home on a movie set. Costume makeup and high quality masks and clothing are quite common.
Costume rental stores can help with putting all this together. A high quality costume can cost a considerable amount of money and renting it might make sense, especially if you want to have a different costume every year.